Clamping relay comparison circuit



June 21, 1960 N. w. BELL 2,942,161

CLAMPING RELAY COMPARISON CIRCUIT Filed June 21, 1957 WTPUT INVENTOR. 4 0970 14/. 55L 4 M/PU T United States Patent CLAMPING RELAY COMPARISON CIRCUIT Norton W. Bell, Monrovia, Calif., assignor to Bell Howell Company, Chicago, Ill., a COI'POl'flilOll of Illinois Filed June 21, 1957, Ser. No. 667,259

1 Claim. (Cl. 317-150) This invention relates generally to a circuit for indicating the polarity or equality of two voltage levels, and more particularly, is concerned with positively clamping a relay in one condition or another, with changes in level of one volt-age with respect to some predetermined fixed level.

It is frequently desirable to provide a circuit which produces one or the other of two output voltage levels in response to an input signal which varies in voltage above and below the level of some reference. An example of this is the so-called inverter circuit, such as a well known Schmidt trigger, commonly used as a logical circuit in computers and the like. I

In slower speed relay-type circuits, a polar relay provides this function. The use of a polar relay to sense polarity or equality in voltage levels is desirable because of the noise-suppressing effect of armature inertia. Thus the polarized relay does not respond to transient etfeets. However, this inertia also limits the time in which a circuit can respond in providing an unambiguous indication of the polarity of the voltage input. The sensitivity of a polarized relay is a function of the signal strength, the smaller the signal the longer the relay takes to respond. This operate time may extend into the period when an unambiguous answer as to the relative polarity of the input is required.

The present invention provides a polarized relay type of comparison circuit responsive to an infinitely variable input voltage of reversible polarity, for producing an output signal of one of two levels depending upon the relative polarity of the input. The circuit insures that the relay will not operate to change the output levels during the time an output is being derived from the comparison circuit. Thus the present circuit provides an output which is unambiguous. This is accomplished by providing an additional clamping winding on the relay which is connected to the output. By means of an independent operating switch coupled between the output terminal and the clamping winding the output voltage thus maintains the polarized relay in one condition regardless of changes in the input. The clamping relay is always connected in when it is desired to read out the output. In this way the polarized relay is clamped and the output maintained in the same condition until the output reading is completed, thus preventing any change in the condition of the polarized relay during the time an output is taken.

For a better understanding of the invention, reference should be had to the accompanying drawing wherein the single figure is a schematic wiring diagram of the comparison circuit.

In the figure, the numeral indicates generally a polarized relay having at least two control windings 12 and 14. As is characteristic of the polarized relay, reversing the current through a control winding reverses the condition of the relay. Moreover, the current need not be maintained through the control winding, as the relay stays biased in one condition or the other due to magnetic remanence in the core of the relay. The relay operates a double-pole single-throw switch 16 by means ice of which an output lead 18 is connected to one end or the other of a grounded center-tap potential source 20. In this manner the output lead 18 is selectively biased to a fixed potential above and below ground reference.

The coil 12 of the polarized relay 10 is normally connected to a source of voltage which may be infinitely variable in magnitude and reversible in polarity. Changes in polarity reverse the flow of current through the control winding 12, which normally tends to reverse the condition of the polarized relay. Due to the inertia of the armature of the switch 16, residual magnetism in the core of the relay, and the inductance of the coil 12, a finite time is required following the reversal of the input voltage before the switch 16 is actuated. The smaller the voltage differential applied across the coil 12, the longer the re lay 10 will take to actuate. If a very small signal is applied to the polar relay, the operate time may extend into the period when an answer as to the polarity of the input is required at the output lead 18.

To prevent this, a second coil, referred to as a clamping coil 14, is provided in the relay which is connected betwen the center point of the potential source 20 and the output lead 18 through a switch 22 which operates independent of both the input and output voltage. When information about the polarity of the input is desired, the switch 22 is closed, completing a circuit through the clamping coil 14. The relay is clamped according to the existing condition of the relay switch 16 at the time the switch 22 is closed. By making the clamping current and hence the magnetic field produced by the clamping coil 14 very much larger than the current normally flowing through and hence the magnetic field produced by the coil 12, the relay is rapidly clamped against further changes in the relay due to the current condition in the coil 12. The coil 14 is connected in relation to the potential of the battery 20 such that the clamping current tends to maintain the polarized relay in its existing condition at the time switch 22 is closed.

What is claimed is:

Apparatus for producing a bistable indication in response to an electrical input signal of variable polarity, said apparatus comprising an input terminal, a polarized relay including a first energizing coil coupled to the input terminal for receiving said input signal and accordingly producing a magnetic field for said relay and also including a dependently operating switching means directionally responsive to said magnetic field, an output terminal, means including said switching means coupled to said output terminal for producing an output voltage of reversible polarity depending upon the condition of the magnetic field of the relay, and means actuated independent of both the input and output voltage for producing a second and stronger magnetic field for said relay which combines with and overcomes that produced by said first coil, said last named means including a second energizing coil on said relay, and including a switch connected between the output terminal and the second coil and actuated independent of both the input and output voltage for connecting the output voltage to said second coil, the second coil being connected in the direction where the polarity of the output voltage causes the coil to maintain the switching means in its condition existing at the time said independently operating switch is closed.

References Cited in the file of this patent UNITED STATES PATENTS 1,599,515 Connery Sept. 14, 1926 1,835,625 Allen Dec. 8, 1931 2,042,109 Lamb May 26, 1936 2,272,914 Keeler Feb. 10, 1942 2,523,717 Ptleger Sept. 26, 1950 

